program Project1;
{$APPTYPE CONSOLE}
var
myStr : String;
i : Integer;
begin
if System.Char.IsLetter('a')
then Console.WriteLine('a is a letter')
else Console.WriteLine('a is not a letter');
myStr := '[AaBb12]'; for i := 0 to Length(myStr)-1 do // Note : 0 based
if System.Char.IsLetter(myStr, i)
then Console.WriteLine(myStr[i+1] + ' is a letter')
else Console.WriteLine(myStr[i+1] + ' is not a letter');
Console.ReadLine;
end.

a is a letter
[ is not a letter
A is a letter
a is a letter
B is a letter
b is a letter
1 is not a letter
2 is not a letter
] is not a letter